This invention is generally in the area of combinatorial chemistry, in particular the use of combinatorial chemistry to optimize Fischer-Tropsch synthesis, primarily to form hydrocarbons in the distillate fuel and/or lube base oil ranges.
The majority of fuel today is derived from crude oil. Crude oil is in limited supply, and fuel derived from crude oil tends to include nitrogen-containing compounds and sulfur-containing compounds, which are believed to cause environmental problems such as acid rain.
Although natural gas includes some nitrogen- and sulfur-containing compounds, methane can be readily isolated in relatively pure form from natural gas using known techniques. Many processes have been developed that can produce fuel compositions from methane. Most of these processes involve the initial conversion of methane to synthesis gas (xe2x80x9csyngasxe2x80x9d).
Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes combustible fuel, among other products. A limitation associated with Fischer-Tropsch chemistry is that it tends to produce a broad spectrum of products, ranging from methane to wax. Product slates for syngas conversion over Fischer-Tropsch catalysts (Fe, Co and Ru) are controlled by polymerization kinetics with fairly constant chain growth probabilities, that fix the possible product distributions. Heavy products with a relatively high selectivity for wax are produced when chain growth probabilities are high. Methane is produced with high selectivity when chain growth probabilities are low.
Methane can be recirculated to ultimately yield combustible liquid fuel. Wax can be processed, for example by hydrocracking and/or hydrotreating followed by oligomerization, to yield combustible liquid fuel. However, it would be advantageous to have new methods for providing a product stream from a Fischer-Tropsch process that has a higher proportion of combustible liquid fuel with less methane to recirculate and/or less wax to process.
Traditional Fischer-Tropsch synthesis has been modified by incorporating an acidic component, such as a zeolite, into the catalyst bed. When C4+ alpha-olefins are produced, the alpha-olefins isomerize to more substituted olefins, cyclize to form aromatics, and/or heavier products are hydrocracked in the presence of the acid catalyst. This reduces the chain growth probability for C4+ and largely minimizes wax formation.
For example, U.S. Pat. No. 4,086,262 to Chang et al. teaches conducting Fischer-Tropsch synthesis with ZSM-5 intimately mixed with the Fischer-Tropsch catalyst. Chang focused on obtaining high octane gasoline (i.e., highly branched hydrocarbons in the gasoline range).
Most work since then has focused on improving the catalyst components and continues to provide highly branched hydrocarbons in the high octane gasoline range. The catalysts are typically iron catalysts, since they operate at higher temperatures where the zeolites tend to be more active. In addition to intimate mixtures of zeolites and Fischer-Tropsch catalysts, some carbon monoxide hydrogenation components have been incorporated directly on zeolites (see, for example, U.S. Pat. No. 4,294,725).
There is a growing interest in developing xe2x80x9cgreenerxe2x80x9d diesel fuels, i.e., fuels which do not contain aromatic, nitrogen or sulfur compounds. Straight chain or slightly branched paraffins in the diesel fuel range tend to have relatively high cetane values. Ideally, such fuels could be provided directly from Fischer-Tropsch reactors if the right combinations of Fischer-Tropsch catalysts and zeolites could be found. However, known combinations of zeolites and Fischer-Tropsch catalysts to date have provided mainly highly branched paraffins in the gasoline range.
It would be advantageous to provide methods for discovering optimum catalyst systems for converting syngas to higher molecular weight products, for example hydrocarbons in the distillate fuel and/or lube base stock base oil ranges. The present invention provides such methods.
The present invention is directed to methods for optimizing the conversion of syngas to hydrocarbons via Fischer-Tropsch synthesis, preferably to form hydrocarbons in the distillate fuel and/or lube base oil ranges. The methods use a combinatorial approach to identify combinations of catalyst systems useful for performing the Fischer-Tropsch reactions. The catalyst combinations include both a Fischer-Tropsch catalyst and a relatively acidic catalyst, for example a molecular sieve, for isomerizing double bonds in C4+ olefins as they are formed. The methods can advantageously be used to generate a database of combinations of catalyst systems and, optionally, reaction conditions, that provide various product streams. As market conditions vary and/or product requirements change, conditions suitable for forming desired products can be identified with little or no downtime using the methods described herein.
Libraries of catalysts suitable for use in a first catalyst system (Fischer-Tropsch catalysts) and a second catalyst system (olefin isomerization catalysts) are prepared. The libraries can optionally include catalysts that possess both types of activity, namely, that can convert syngas to olefins and also that isomerize the olefins.
The catalysts are preferably combined in a logical manner, for example in an Axc3x97B array, where each position in the A column includes one or more catalysts from the first catalyst system, and each position in the B row includes one or more catalysts from the second catalyst system. In this manner, virtually every possible combination of catalysts in the libraries can be evaluated. The combinations of catalysts can be evaluated using varied reaction conditions, which can provide a) a combinatorial library of product streams and a database including the combination of catalysts and reaction conditions to provide each product stream and/or b) the optimum combination of catalysts and reaction conditions for obtaining a desired product stream.
In addition to catalyst composition and reaction conditions, a third set of variables with great influence on the catalytic activity/selectivity is the manner in which various xe2x80x9cpre-treatmentxe2x80x9d steps are carried out. Such pre-treatment variations include the time and temperature of catalyst washing; heating rate, hold time, hold temperature, and relative humidity during drying. The same pre-treatments can be varied during catalyst calcining, reduction, and activation. In catalyst reduction, the hydrogen content and total pressure can also be varied, as can the pressure and CO partial pressure during activation in CO. Other reduction methods can also be used, including as treatment with citrate, alcohols, and metal hydrides. Additional pre-treatments can also be performed, including modifying the acidity using acid leaching, base titration, ion exchange, vapor deposition of Si or Al species and steaming. These pre-treatments can be done individually or in combination on the individual catalyst components or on the composite. These different pre-treatments can be used with the different catalyst compositions to increase the size of the catalyst libraries. Alternatively, a single catalyst can be subjected to a plurality of different pre-treatments, or a library of catalysts can be subjected to a single pre-treatment, with the process repeated as desired.
The products can include olefins such as ethylene, iso-paraffins, and combinations thereof, and preferably include iso-paraffins in the distillate fuel and/or lube base stock ranges, and, more preferably, iso-paraffins in the jet or diesel range.